1. Field of Endeavor
The present invention deals with the field of workpiece machining. It relates to a process for producing a body provided with a slot as a test crack, in particular, a reference body for the nondestructive testing of materials.
2. Brief Description of the Related Art
Nondestructive inspections of components which are subject to high levels of loading and are susceptible to cracking are of major importance for power plants which are operated for relatively long periods, in order to detect incipient damage in good time and to take countermeasures in order to ensure high overall availability. In order to make it possible to calibrate the apparatus and processes for the nondestructive testing of materials and detection of cracks used for these inspections, e.g., ultrasound processes, use is made of test bodies with artificially produced crack-like structures, i.e., test cracks.
Since the cracks which occur naturally are very narrow and the widths of these cracks are typically in the sub-millimeter range, it is not possible to produce test cracks such as these using conventional material machining methods such as, for example, drilling or milling. To date, therefore, the standard method used for the artificial production of cracks has been the spark erosion process (electrical discharge machining or EDM). For small crack-like structures, use is made, in particular, of the plunge erosion process (sinker EDM) in which the workpiece to be machined is immersed in a dielectric liquid such as, for example, oil and machined using an electrode, the shape of which corresponds to the recess to be produced in the workpiece. A power supply produces an electrical potential difference between the workpiece and electrode. If the electrode approaches the workpiece, the insulation provided by the dielectric liquid breaks down and a spark skips between the electrode and workpiece. The heat and cavitation thereby produced locally vaporize the material of the workpiece (and, to a certain extent, of the electrode as well), and this provides a continuous material removal process.
However, a critical limitation of the EDM process is that it can be used only for electrically conductive materials, which are mostly iron alloys. EDM can cut small or unusually shaped angles, complicated contours or recesses in hardened steel without the need for heat treatments for soft-annealing or renewed hardening, and it can also be used for exotic materials such as titanium, Hastelloy®, Kovar® and Inconel®.
However, the limitation with respect to the size of the workpieces is particularly critical in terms of the production of test cracks for the nondestructive testing of materials by EDM. Since the workpieces have to be immersed in a bath of a dielectric liquid, it is not possible to machine large and heavy structures such as, for example, large forged pieces or pipes in this way.
On the other hand, it is known to machine workpieces by using an abrasive, high-pressure water jet (abrasive water jet or AWJ) which can make cuts in a material such as, for example, a stainless steel sheet or the like (see document U.S. Pat. No. 5,018,317). A water jet such as this may also be used at inaccessible locations in order to remove pins from machinery (see document WO-A1-2008/083889). Finally, it is possible (see document U.S. Pat. No. 5,704,824) to use an AWJ apparatus first to produce a resistant mask and then to carry out material-removing machining on a workpiece through this mask.